1. Field of the Invention
The present invention relates generally to the suppression of tube waves within a bore hole and, more particularly, to an apparatus and method for suppressing or attenuating tube waves within a bore hole at increased depths and/or pressures including automatically adjusting internal pressure of a wave suppressing apparatus responsive to local bore hole pressure.
2. State of the Art
Seismic surveys are conducted in various ways, including surface and subsurface techniques. Surface seismic techniques generally include placing both a seismic energy source, such as an air gun, explosive source or impact-type, vibrational seismic device, and one or more seismic energy detectors, such as, for example, geophones, at the surface of the earth above a subterranean formation, the characteristics of which are to be obtained. The seismic energy source induces wave energy into the formation. The response of the wave energy, as it is reflected/transmitted back to the surface, is detected and recorded by the seismic detectors, also termed receivers. The response of the wave energy is analyzed so that the characteristics of the subterranean formation may be determined and mapped.
In subsurface processes, various methods are used. For example, in vertical seismic profiling (VSP) the seismic energy source remains at the surface while the seismic detectors are located within a bore hole, which may also be referred to herein as a bore hole, formed in the subterranean formation of interest. In inverse VSP processes the seismic energy source is located within the bore hole while the seismic detectors are located at the surface.
Another subsurface process, known as cross-well seismic profiling, includes positioning the seismic energy source in a first borehole and then positioning seismic detectors in one or more laterally adjacent boreholes formed in the general proximity of the subterranean formation of interest. VSP, inverse VSP and cross-well seismic profiling have been generally noted as providing greater resolution than surface techniques as such processes are able to make use of direct and/or refracted wave fields traveling through the various subterranean strata rather than reflected wave fields only.
Yet another subsurface process which has more recently been under development may be referred to as single well seismic profiling. Single well seismic profiling includes disposing both the seismic energy source and the seismic detectors within the same bore hole. Thus, single well seismic profiling inherently deals with reflective wave fields, but allows a closer look at the surrounding formation as the seismic energy source and detectors may be disposed at various elevations within the bore hole to map the formation at greater depths than is possible using surface profiling. Additionally, single well seismic profiling may be considerably less expensive and time consuming than cross-well seismic profiling as only a single bore hole must be drilled. Further, in some formations which are of interest, potential suitable locations for multiple bore holes may be limited, thereby eliminating the possibility of using cross-well seismic profiling.
One difficulty encountered when using subsurface profiling techniques, in either cross-well or single well seismic profiling, is the generation of tube waves, sometimes referred to as Stoneley waves. Tube waves are basically the result of wave energy transmitted to the bore hole fluid via the surrounding formation or directly from a source in the same well. Tube waves propagate up and down the bore hole through fluid contained therein with the bore hole wall or casing acting as a wave guide. Tube waves typically travel through the bore hole with little or no attenuation, the wave energy being substantially reflected at the upper and lower ends of the borehole or at any other discontinuity within the bore hole. Such waves interfere with the primary wave fields being detected and analyzed, potentially compromising the survey being performed and, at the very least, complicating the process of analyzing the wave energy which is detected.
Suppression or attenuation of tube waves significantly enhances the signal-to-noise ratios attainable in bore hole environments thereby reducing the interference or masking effect of tube waves with respect to the seismic wave signals of interest. Thus, various techniques have been implemented, with varying degrees of success, in an effort to suppress tube waves. For example, plugs or packers have been strategically placed within the bore hole in an attempt to reduce or eliminate the amplitude of the tube wave and specified locations. However, such plugs and packers are of limited effect as they require secure clamping to the bore hole wall or casing thereby introducing mechanical complexities as well as providing a path for wave energy to be transferred to the bore hole wall or casing, resulting in a possible secondary wave source.
Another method of suppressing tube waves includes positioning a gas filled bladder within the bore hole. The bladder acts to absorb and attenuate wave energy as the tube wave propagates thereby. For example, U.S. Pat. No. 4,858,718 to Chelminski provides an apparatus which includes a gas filled bladder coupled with a gas source. The gas source may be located at the surface of the bore hole, or alternatively, may include a precharged vessel which is disposed within the bore hole along with the bladder. Gas is supplied to the bladder via a pressure reducing valve so as to maintain a pressure within the bladder which is greater than the pressure of the surrounding fluid as the bladder descends to greater depths within the bore hole. However, in order to go to significant depths, the attenuation device of Chelminksi must either be supplied with pressure from the surface, meaning that high pressure tubing must be run down the bore hole with the attenuation device, or must incorporate a pressure vessel rated to withstand extreme pressures and provide high pressure gas for deployment in the bore hole. Use of such a pressure vessel significantly increases the cost of such an attenuation device, increases the size, weight and complexity thereof, and also introduces the potential for danger to personnel and equipment at the surface through the use of extreme pressurization equipment.
In view of the shortcomings in the state of the art, it would be advantageous to provide an apparatus and method for the attenuation of tube waves at increased depth which is autonomous (e.g., does not require input or control from the surface) while also minimizing the size and rating of any pressure vessel required for use therewith.